1. Field of the Invention
The present invention relates to a fluid bearing apparatus, and more particularly, to a fluid bearing apparatus in which a thrust load supporting portion is formed between an external side of a fixing shaft and an internal side of a bushing, which faces the fixing shaft.
2. Description of the Related Art
Recently, with the improvement of technologies in the information and computer industries, there has been a trend to provide high speed driving motors with rotary shafts which are highly accurate, have no oscillation and provide superior high speed rotation performance. Driving motors of this type are required for various machines, such as a polygon mirror driving gear of a laser printer, a spindle motor of a hard disk, a head driving motor of a VCR, and the like. In this respect, a driving motor capable of stably rotating at high speed has been developed together with a fluid bearing apparatus, which enables the rotary shaft of the driving motor to rotate at high speed with high accuracy.
A conventional fluid bearing apparatus which is mainly used in a polygon mirror driving gear of a laser printer is shown in FIG. 1.
Referring to FIG. 1, a polygon mirror 10 mounted on a thrust bearing 25 reflects laser beams onto a photosensitive drum. A driving gear for rotating the polygon mirror 10 at a predetermined revolutions per minute (rpm) includes a bushing 30 and a plate 50. The thrust bearing 25 is fixed to the bushing 30. A fixing shaft 40 is inserted into a through hole of the bushing 30, and the outer surface of the shaft 40 is separated from the inner wall of bushing 30 by a predetermined amount. On the outside of the fixing shaft 40, a herring bone shaped first kinetic pressure generating groove 45 is formed. In the thrust bearing 25 which faces an upper end portion of the fixing shaft 40, a second kinetic pressure generating groove 20 is formed.
The operation of the fluid bearing apparatus as aforementioned will now be described below.
If the power source is applied to a stator (not shown), the plate 50 rotates by electromagnetic action between the stator and a rotor (not shown) onto which the plate 50 is mounted. The plate 50 and the bushing 30, which is combined with the plate 50, rotate. Also, the thrust bearing 25 and the polygon mirror 10 rotate together. At this time, a fluid flows into a fluid inlet groove 45a of the first kinetic pressure generating groove 45 in the fixing shaft 40, so that a fluid pressure occurs. As a result, a predetermined clearance is formed between the fixing shaft 40 and the bushing 30 to support a radial load caused by rotation.
The fluid flows into a center portion on the upper end portion of the fixing shaft 40 through the second kinetic pressure generating groove 20 from edge portions on the upper end portion of the fixing shaft 40, so that the fluid pressure occurs. A minimum fluid pressure P.sub.min which boosts the thrust bearing 25 occurs as the rotative speed increases. The thrust bearing 25 rotates without contacting with the fixing shaft 40.
However, the conventional fluid bearing apparatus has several problems.
That is, in order for the thrust bearing 25 to rotate without contacting with the fixing shaft 40, a predetermined rotative speed of the thrust bearing 25 is required. In particular, there must separately be provided a thrust bearing in which a second kinetic pressure generating groove is formed to support a thrust load which occurs in the polygon mirror.